This invention relates generally to ionization detection in an ignition system and more particularly to ionization detection in an ignition system using a buffered ionization sensing current source.
The relationship between spark plug gap ionization and engine misfire is well understood in the automotive industry. As such, it is well known that following a successful ignition electrical conductivity within a spark plug gap increases due to the ionization of hot combustion gases. Thus, if a current, specifically an ionization current, could be generated from the ionization of these hot combustion gases, this ionization current could be used to gather valuable information regarding the combustion process. Measurement of this ionization current could provide information relating to engine misfire, engine knock, spark plug fouling, approximate fuel/air ratios as well as many other combustion characteristics.
As such, ionization current detection in an ignition system is used to determine information regarding the combustion process. As discussed above, when a spark plug sparks, gases surrounding the spark plug gap ignite causing these gases to become ionized and increasing the electrical conductivity within the gap. At this point, application of a voltage across the gap results in a current, specifically an ionization current, which can then be measured. Typically, this voltage is applied using a voltage source and the ionization current is measured via measuring electronics located in the Engine Control Module (ECM) or some other remote location.
In some ion sensing ignition systems, the measuring electronics are remotely located away from the spark plug and the ignition coil, effectively putting the measuring electronics at a different ground potential than the spark plug and the ignition coil. It should be noted that although the measuring electronics and the spark plug and the ignition coil have different ground potentials, they are ohmically communicated with each other through a common system ground. However, because they do not share the same ground voltage potential they effectively do not share a common ground and because the measuring electronics and the spark plug do not share a common ground, the ion sensing system may experience dynamic ground potential differences. When the measuring electronics ground potential changes relative to the spark plug ground potential a small distortion voltage is created with respect to the measuring electronics ground. This small distortion voltage is problematic because the ionization current levels are very small making the system very sensitive to any dynamic ground differences. In fact, because the ionization current levels are so small any distortion can become significant. As an example, this distortion can be especially problematic if the ECM is attempting to extract small amplitude engine knock information from the ionization current.
Currently, there are a few approaches available to resolve the effects created by these dynamic ground potential differences. One approach is to mount the ECM directly to the engine. This approach is proven effective and works to minimize any ground differences between the ECM and the engine. However, this approach can be expensive due to the fact that the ECM would have to survive high engine temperatures and engine vibration levels.
A second approach would be to use differential amplifiers at the input of the ECM. Although this is possible and could be effective, this approach has a few drawbacks. First, the differential amplifier could be expensive and subject to drift with age and temperature. Second, because the ground difference can be both negative and positive the differential amplifier would require a negative power supply. Third, the differential amplifier would have a signal input and a ground sense input requiring additional leads.
Lastly, a third approach would be to put the signal processing circuitry in the ignition coil. This approach should be highly effective and eliminate any potential ground differences. However, this approach could be expensive because it would require communicating the signal information from the ignition coil to the ECM taking into account the varying ground potential differences. Although this information can be communicated using many different methods, such as digital encoding and pulse width encoding, complex logic circuitry would be required in each ignition coil. Because the ignition coil is mounted on the engine, the complex logic circuitry would have to be able to survive high engine temperatures and engine vibration levels. Finally, having this logic circuitry in each coil will tend to limit the signal processing capability due to size, temperature and cost.
Therefore, it is considered advantageous to provide an ionization current detection circuit design that utilizes a buffered ion sense current source at the output of an ion sense ignition coil so as to cause the detected ionization current to not be sensitive to voltage differences between engine ground and ECM ground.
In an ignition coil assembly of an ion sensing ignition system having an ignition coil output, a buffered ion-sense current source circuit comprising: a current sensing circuit, the current sensing circuit being disposed so as to be communicated with the ignition coil output; and an active current source circuit, the active current source circuit being disposed so as to be communicated with the current sensing circuit and a current measuring device.